1. Field of the Invention
The present invention relates to yellowing prevention technology in articles having a microporous body part (printing media such as a medium on which a photographic image is formed or a label on which ordinary printing is performed; or articles having, or partially having, an elastic microporous body or microporous body which is capable of swelling). In particular, the present invention also relates to a recording medium, and production method thereof, which has high-image quality and is capable of preventing yellowing over a long period of time, that is suitable for ink recording using aqueous inks.
The present invention also relates to a method for diffusing a sulfur-containing organic acid into a microporous layer, and a method for producing a recording medium for application in ink recording which uses an aqueous ink employing such diffusion method.
The present invention especially relates to an inkjet recording medium having a porous part composed of hydrated alumina, and a production method thereof, which does not impair printing quality after printing, has long-term image shelf-life, functions to prevent yellowing of a white-background during storage in a file holder, and can maintain yellowing prevention performance for at least a distribution storage period corresponding to a product life between production of the medium and printing in the medium is printed.
2. Related Background Art
Articles having microporous body parts are employed in a large number of fields. The inkjet recording field can be cited as a representative example. An inkjet recording method performs high image quality printing of images, characters or the like by employing a range of operating principles to cause microdroplets of a liquid for recording (recording liquid), such as an ink, to fly up and adhere onto a recording medium having a microporous body part. The recent spread of digital cameras, digital video, scanners, personal computers and other such devices has expanded the demand for printers which employ an inkjet recording system having such characteristics.
However, in the ink recording field, while printing quality is obviously demanded, also being demanded are maintenance of the white-background yellowing prevention effects during long-term storage in a resin file holder after printing, and a capability for preventing yellowing at least during the distribution period that is required for overseas ocean transport after production. Therefore, for inkjet recording media, characteristics of a high drying speed, excellent color developability of the colorant, high surface gloss and enabling image formation with a high resolution are being demanded. As a recording medium capable of providing an image comparable to a silver halide photograph, an inkjet recording medium has been realized which is composed of a fine inorganic pigment such as silica or alumina, and a binder for such a pigment, and in which a microporous body part with a high porosity as an ink receiving layer is layered on a support.
Japanese Patent Application Laid-Open No. H07-232475 discloses that hydrated alumina is preferable as a material to be used for an ink receiving layer, as dye adhesion in the ink is good as a result of hydrated alumina having a positive charge, and an image having excellent color developability can be attained. Hydrated alumina having a boehmite structure is disclosed as being more preferable due to its good dye adsorption properties, ink absorption properties and transparency.
However, a conventional recording medium provided with a porous site comprising a large number of microvoids as the ink receiving layer can sometimes undergo yellowing of the white background portion of the image over time if stored in some kind of file holder. It is known that since the microporous body part of a recording medium having such a structure possesses a large number of microvoids, if a medium is placed in a resin file and stored, a phenolic antioxidant, as represented by BHT (2,6-di-t-butyl-p-methylphenol) adheres to the ink receiving layer of the recording medium from the file holder, and is gradually oxidized to form a yellow oxide, whereby yellowing occurs. Regarding such yellowing due to BHT, documents such as “Polymer Degradation and Stability 50 (1995) 313-317”, “Textil Praxis International Oktober (1980) 1213-1215”, “Textile Chemist and Colorist April (1983) Vol. 15 No. 4 52-56” and “Text. Progr. 15 (1987) 16” disclose that the phenolic antioxidant oxidizes to a quinone methide structure, which then dimerizes and again oxidizes to form a yellow compound having a stilbene quinone structure.
Japanese Patent Application Laid-Open No. H11-34484 discloses an invention in which an organic acid or inorganic acid, such as maleic acid or hydrochloric acid is used as a yellowing countermeasure, to set the surface and interior pH of the ink receiving layer comprising hydrated alumina which has a pseudoboehmite structure in a range of between 4.0 and 5.4, inclusive thereof. Japanese Patent Application Laid-Open Nos. 2003-1931 and 2002-96546 describe a recording medium which comprises an ink receiving layer (containing silica) layered on a non-water-absorbent support, and a coating layer with a film surface pH adjusted to 4.2, the coating layer comprising a sulfur-containing organic compound such as a thioether compound, thiourea compound, disulfide compound, mercapto compound, sulfinic acid compound, or thiosulfonic acid compound, but comprising simply “an acid” in Examples, actually. In these documents, the film surface pH is given as between 2 and 6, preferably 3 and 5, although the reason for this is not clear.
Japanese Patent Application Laid-Open No. 2003-1931 describes a production method of an ink absorbing layer which contains a larger quantity of the above-described sulfur-containing organic compound in its lower layer portion than in its upper layer portion. Japanese Patent Application Laid-Open No. 2003-1931 further describes a production method for the same ink absorbing layer which consists of coating a non-water-absorbing support with a coating solution comprising a binder and the sulfur-containing organic compound, and a coating solution comprising inorganic fine particles and a binder. Japanese Patent Application Laid-Open No. 2003-1931 also discloses that since ink absorption properties deteriorate if the added amount of the acid for attaining a pH of 4.2 is increased, the sulfur-containing organic compound is added such that it does not contain any additives or in an amount of 0.1 to 0.3 g/m2.
Japanese Patent Application Laid-Open No. 2002-96546 describes a recording material provided with an ink absorbing layer on a non-water-absorbing support and an upper layer which comprises the above-described sulfur-containing organic compound, wherein the sulfur-containing organic compound is present in a high concentration in the upper portion of the ink receiving layer. As production methods for such material, Japanese Patent Application Laid-Open No. 2002-96546 discloses a method which provides an ink absorbing layer onto a non-water-absorbing support and then coats an aqueous solution containing the above sulfur-containing organic compound; and a method which coats an aqueous solution containing silica and the above sulfur-containing organic compound onto an ink absorbing layer.
Japanese Patent Application Laid-Open No. 2003-291513 discloses the addition of an ammonium salt and acid for the purpose of improving yellowing resistance in the ink receiving layer. It is disclosed that the ammonium salt and acid are caused to fly up due to the drying during coating after a water-soluble resin for forming a colorant receiving layer has been cured, whereby as a result anions of the acid are caused to remain in the system, so that the pH of the colorant image receiving (colorant receiving) layer is reduced. Japanese Patent Application Laid-Open No. 2003-291513 also describes an inkjet recording sheet prepared by coating as a pigment 200 ml of a pH 3.5 coating solution which consists of a silica particulate, polyvinyl alcohol, boric acid, polyoxyethylenelauryl ether and ion-exchange water onto a colorant receiving layer, drying the coating solution, and then coating 20 g/m2 of a coating solution to which sulfonic acid was added in an ammonium chloride salt and acid state. However, in this document, the oxidized ratio of retained BHT is simply decreased by the addition of the ammonium chloride salt and acid.